Introduction

This documentation attempts to explain object inheritance and how it can be used in your object definitions.

If you are confused about how recursion and inheritance work after reading this, take a look at the sample object config files provided in the Nagios distribution. If that still doesn't help, drop an email message with a detailed description of your problem to the nagios core support forum.

Basics

There are three variables affecting recursion and inheritance that are present in all object definitions. They are indicated in red as follows:

define someobjecttype {
    object-specific variables ...
    name      template_name
    use       name_of_template_to_use
    register  [0/1]
}

The first variable is name. Its just a "template" name that can be referenced in other object definitions so they can inherit the objects properties/variables. Template names must be unique amongst objects of the same type, so you can't have two or more host definitions that have "hosttemplate" as their template name.

The second variable is use. This is where you specify the name of the template object that you want to inherit properties/variables from. The name you specify for this variable must be defined as another object's template named (using the name variable).

The third variable is register. This variable is used to indicate whether or not the object definition should be "registered" with Nagios. By default, all object definitions are registered. If you are using a partial object definition as a template, you would want to prevent it from being registered (an example of this is provided later). Values are as follows: 0 = do NOT register object definition, 1 = register object definition (this is the default). This variable is NOT inherited; every (partial) object definition used as a template must explicitly set the register directive to be 0. This prevents the need to override an inherited register directive with a value of 1 for every object that should be registered.

Local Variables vs. Inherited Variables

One important thing to understand with inheritance is that "local" object variables always take precedence over variables defined in the template object. Take a look at the following example of two host definitions (not all required variables have been supplied):

define host {
    host_name               bighost1
    check_command           check-host-alive
    notification_options    d,u,r
    max_check_attempts      5
    name                    hosttemplate1
}

define host {
    host_name               bighost2
    max_check_attempts      3
    use                     hosttemplate1
}

You'll note that the definition for host bighost1 has been defined as having hosttemplate1 as its template name. The definition for host bighost2 is using the definition of bighost1 as its template object. Once Nagios processes this data, the resulting definition of host bighost2 would be equivalent to this definition:

define host {
    host_name               bighost2
    check_command           check-host-alive
    notification_options    d,u,r
    max_check_attempts      3
}

You can see that the check_command and notification_options variables were inherited from the template object (where host bighost1 was defined). However, the host_name and max_check_attempts variables were not inherited from the template object because they were defined locally. Remember, locally defined variables override variables that would normally be inherited from a template object. That should be a fairly easy concept to understand.

Tip: If you would like local string variables to be appended to inherited string values, you can do so. Read more about how to accomplish this below.

Inheritance Chaining

Objects can inherit properties/variables from multiple levels of template objects. Take the following example:

define host {
    host_name               bighost1
    check_command           check-host-alive
    notification_options    d,u,r
    max_check_attempts      5
    name                    hosttemplate1
}

define host {
    host_name               bighost2
    max_check_attempts      3
    use                     hosttemplate1
    name                    hosttemplate2
}

define host {
    host_name     bighost3
    use           hosttemplate2
}

You'll notice that the definition of host bighost3 inherits variables from the definition of host bighost2, which in turn inherits variables from the definition of host bighost1. Once Nagios processes this configuration data, the resulting host definitions are equivalent to the following:

define host {
    host_name               bighost1
    check_command           check-host-alive
    notification_options    d,u,r
    max_check_attempts      5
}

define host {
    host_name               bighost2
    check_command           check-host-alive
    notification_options    d,u,r
    max_check_attempts      3
}

define host {
    host_name               bighost3
    check_command           check-host-alive
    notification_options    d,u,r
    max_check_attempts      3
}

There is no inherent limit on how "deep" inheritance can go, but you'll probably want to limit yourself to at most a few levels in order to maintain sanity.

Using Incomplete Object Definitions as Templates

It is possible to use incomplete object definitions as templates for use by other object definitions. By "incomplete" definition, I mean that all required variables in the object have not been supplied in the object definition. It may sound odd to use incomplete definitions as templates, but it is in fact recommended that you use them. They can serve as a set of defaults for use in all other object definitions. Take the following example:

define host {
    check_command           check-host-alive
    notification_options    d,u,r
    max_check_attempts      5
    name                    generichosttemplate
    register                0
}

define host {
    host_name       bighost1
    address         192.168.1.3
    use             generichosttemplate
}

define host {
    host_name       bighost2
    address         192.168.1.4
    use             generichosttemplate
}

Notice that the first host definition is incomplete because it is missing the required host_name variable. We don't need to supply a host name because we just want to use this definition as a generic host template. In order to prevent this definition from being registered with Nagios as a normal host, we set the register variable to 0.

The definitions of hosts bighost1 and bighost2 inherit their values from the generic host definition. The only variable we've chosen to override is the address variable. This means that both hosts will have the exact same properties, except for their host_name and address variables. Once Nagios processes the config data in the example, the resulting host definitions would be equivalent to specifying the following:

define host {
    host_name               bighost1
    address                 192.168.1.3
    check_command           check-host-alive
    notification_options    d,u,r
    max_check_attempts      5
}

define host {
    host_name               bighost2
    address                 192.168.1.4
    check_command           check-host-alive
    notification_options    d,u,r
    max_check_attempts      5
}

At the very least, using a template definition for default variables will save you a lot of typing. It'll also save you a lot of headaches later if you want to change the default values of variables for a large number of hosts.

Custom Object Variables

Any custom object variables that you define in your host, service, or contact definition templates will be inherited just like other standard variables. Take the following example:

define host {
    _customvar1         somevalue   ; <-- Custom host variable
    _snmp_community     public      ; <-- Custom host variable
    name                generichosttemplate
    register            0
}

define host {
    host_name       bighost1
    address         192.168.1.3
    use           generichosttemplate
}

The host bighost1 will inherit the custom host variables _customvar1 and _snmp_community, as well as their respective values, from the generichosttemplate definition. The effective result is a definition for bighost1 that looks like this:

define host {
    host_name           bighost1
    address             192.168.1.3
    _customvar1         somevalue
    _snmp_community     public
}

Cancelling Inheritance of String Values

In some cases you may not want your host, service, or contact definitions to inherit values of string variables from the templates they reference. If this is the case, you can specify "null" (without quotes) as the value of the variable that you do not want to inherit. Take the following example:

define host {
    event_handler       my-event-handler-command
    name                generichosttemplate
    register            0
}

define host {
    host_name       bighost1
    address         192.168.1.3
    event_handler   null
    use             generichosttemplate
}

In this case, the host bighost1 will not inherit the value of the event_handler variable that is defined in the generichosttemplate. The resulting effective definition of bighost1 is the following:

define host {
    host_name   bighost1
    address     192.168.1.3
}

Additive Inheritance of String Values

Nagios Core gives preference to local variables instead of values inherited from templates. In most cases local variable values override those that are defined in templates. In some cases it makes sense to allow Nagios Core to use the values of inherited and local variables together.

This "additive inheritance" can be accomplished by prepending the local variable value with a plus sign (+). This features is only available for standard (non-custom) variables that contain string values. Take the following example:

define host {
    hostgroups      all-servers
    name            generichosttemplate
    register        0
}

define host {
    host_name       linuxserver1
    hostgroups      +linux-servers,web-servers
    use             generichosttemplate
}

In this case, the host linuxserver1 will append the value of its local hostgroups variable to that from generichosttemplate. The resulting effective definition of linuxserver1 is the following:

define host {
    host_name       linuxserver1
    hostgroups      all-servers,linux-servers,web-servers
}

Implied Inheritance

Normally you have to either explicitly specify the value of a required variable in an object definition or inherit it from a template. There are a few exceptions to this rule, where Nagios will assume that you want to use a value that instead comes from a related object. For example, the values of some service variables will be copied from the host the service is associated with if you don't otherwise specify them.

The following table lists the object variables that will be implicitly inherited from related objects if you don't explicitly specify their value in your object definition or inherit them from a template. For example, if you add a contact to a service, it will no longer inherit contacts or contact_groups from the host.

Implied/Additive Inheritance in Escalations

Service and host escalation definitions can make use of a special rule that combines the features of implied and additive inheritance. If escalations 1) do not inherit the values of their contact_groups or contacts directives from another escalation template and 2) their contact_groups or contacts directives begin with a plus sign (+), then the values of their corresponding host or service definition's contact_groups or contacts directives will be used in the additive inheritance logic.

Here's an example:

define host {
    name            linux-server
    contact_groups  linux-admins
    ...
}

define hostescalation {
    host_name       linux-server
    contact_groups  +management
    ...
}

This is a much simpler equivalent to:

define hostescalation {
    host_name       linux-server
    contact_groups  linux-admins,management
    ...
}

Important values

Service templates can make use of a special rule which gives precedence to their check_command value. If the check_command is prefixed with an exclamation mark (!), then the template's check_command is marked as important and will be used over the check_command defined for the service (this is styled after CSS syntax, which uses ! as an important attribute).

Why is this useful? It is mainly useful when setting a different check_command for distributed systems. You may want to set a freshness threshold and a check_command that forces the service into a failed state, but this doesn't work with the normal templating system. Using this important flag allows the custom check_command to be written, but a general distributed template can be used to overrule the check_command when used on a central Nagios server.

For instance:

# On master
define service {
    name                    service-distributed
    register                0
    active_checks_enabled   0
    check_freshness         1
    check_command           !set_to_stale
}

# On slave
define service {
    name                    service-distributed
    register                0
    active_checks_enabled   1
}

# Service definition, used by master and slave
define service {
    host_name               host1
    service_description     serviceA
    check_command           check_http...
    use                     service-distributed
    ...
}

Multiple Inheritance Sources

Thus far, all examples of inheritance have shown object definitions inheriting variables/values from just a single source. You are also able to inherit variables/values from multiple sources for more complex configurations, as shown below.

# Generic host template
define host {
    name                    generic-host
    active_checks_enabled   1
    check_interval          10
    ...
    register                0
}

# Development web server template
define host {
    name                    development-server
    check_interval          15
    notification_options    d,u,r
    ...
    register                0
}

# Development web server
define host {
    use             generic-host,development-server
    host_name       devweb1
    ...
}

In the example above, devweb1 is inheriting variables/values from two sources: generic-host and development-server. You'll notice that a check_interval variable is defined in both sources. Since generic-host was the first template specified in devweb1's use directive, its value for the check_interval variable is inherited by the devweb1 host. After inheritance, the effective definition of devweb1 would be as follows:

# Development web server
define host {
    host_name               devweb1
    active_checks_enabled   1
    check_interval          10
    notification_options    d,u,r
    ...
}

Precedence With Multiple Inheritance Sources

When you use multiple inheritance sources, it is important to know how Nagios handles variables that are defined in multiple sources. In these cases Nagios will use the variable/value from the first source that is specified in the use directive. Since inheritance sources can themselves inherit variables/values from one or more other sources, it can get tricky to figure out what variable/value pairs take precedence.

Consider the following host definition that references three templates:

# Development web server
define host{
    use         1, 4, 8
    host_name       devweb1
    ...
    }

If some of those referenced templates themselves inherit variables/values from one or more other templates, the precendence rules are shown to the right.

Testing, trial, and error will help you better understand exactly how things work in complex inheritance situations like this.